Bone mineral is basically calcium phosphate, and both elements (Ca and P) are required for bone acquisition. Typical Ca intakes in the U.S. are lower than current recommendations, and typical P intakes, higher. Thus attention has been focused mainly on increasing Ca intake, by supplementation and food fortification. The salt most commonly used for these purposes is calcium carbonate. But while the average P intake tends to be adequate or generous, nevertheless substantial proportions of older women ingest less than 70 percent of the RDA for phosphorus on any given day, and hence may be said to be at risk for P deficiency. When these women are given a combination of anabolic therapy and supplemental Ca (as the carbonate), the added Ca may uncover and aggravate the latent P deficiency. This is both because Ca binds phosphate in the gut and reduces its absorption (thereby effectively lowering the P intake still further) and because the induced bone anabolism will itself consume phosphorus, as a result of which absorbed phosphorus may not be sufficient to support the increase in bone mineral mass made possible by this therapy. To test the possible importance and value of supplementing both of the components of bone mineral in support of anabolic therapy of osteoporosis, we propose a 1-year randomized trial, comparing, in two groups of teriparatide-treated postmenopausal osteoporotic women, calcium supplements with and without extra phosphorus (i.e., Ca phosphate vs. Ca carbonate). The principal outcome measure will be change in bone mineral content over the one year of the trial. A secondary outcome is measurement of bone resorption biomarkers so as to assess whether the phosphate salt elevates remodeling relative to the carbonate salt. A finding of superiority of the phosphate-containing Ca supplement would provide evidence leading to a cost-neutral change in Ca sources and a corresponding improvement in osteoporosis co-therapy (and possibly osteoporosis prophylaxis as well).